Radio receiving system



p 0; 9 5- w. A. MaCDONALD 2,01 ,086

RADIO RECEIVING SYSTEM Filed May 26, 192? INVENTOR Patented Sept. 10, 1935 UNITED STATES RADIO RECEIVING SYSTEM William a. MacDonald, Little Neck, N. Y., assignor to Bazeltine Corporation, Jersey City, N. 1., a corporation of Delaware Application May as, 1925, Serial No. 32,889

'32 Claims. (01. 350-20) This invention relates to radio receiving systems, and has for its object the provision of an electron relay coupling device whereby the input circuit of the system is caused to be independent of the natural period of the antenna system. Prevention of radiation and other features are also realized by proper application of this invention.

' In brief, e invention provides such an arrangement of paratus that the antenna is connected to the i ut circuit of a thermionic relay or vacuum tub utilized as an antenna coupling device w usual practice would be employed in a double capacity, as is more clearly described hereinafter.

The question of providing a satisfactory antenna coupling device for radio receivers has presented a numberof requirements, the more important of which are first, a coupling device which substantially prevents radiation from vacuum tubes oscillating within the receiver, and second, a coupling device which allows the receiver to function independently of the natural period of the antenna; Both of these desirable features are, together with others, provided by this invention, as will now be described in detail in connection with the accompanying drawing which is a circuit diagram of a. radio receiver comprising two stages of tuned radio-frequency amplification, a detector and two stages of audiofrequency amplification. Except for the novel method of connecting the antenna and concurrent means for transferring radio-frequency energy, the receiver connections illustrated are I those of the now well-known radio receiving system employing tuned radio-frequency amplification in which the undesirable capacity coupling effectively between the grid and plate electrodes, together with the connections thereto, of the radio-frequency amplifying tubes is neutralized by means of the inventions described in Hazeltine U. S. Patents 'Nos. 1,489,228 and 1,533,- 858.

Referring to the drawing: Fig. 1 is a circuit diagram of a radio receiver comprising two radio-frequency amplifying stages, a vacuum tube detector and twoaudiofrequency amplifying stages, the antenna system being coupled to the receiver in accordance with the present invention.

Fig. 2 is a modification of the arrangement of vFig. 1 wherein the radio-frequency amplifying stages are omitted, and the detector includes means whereby it may be caused to be regenerative. I

Fig. 3 shows an alternative radio-frequency coupling device which may be" substituted in either of the previously illustrated systems in place of a radio-frequency transformer.

In Fig. 1, the antenna system, comprising ans tenna l and ground 2, instead of being connected to the input of the first radio-frequency or highfrequency amplifying tube V1, as has been the custom heretofore, is connected to the input circuit of the first audio-frequency or low frequency 10 amplifying tube V4. Thus tube V4, in addition .to functioning in the first stage of audio-frequency amplification, also acts as a substantially untuned radio-frequency amplifying and coupling device between the antenna and the input 15 circuit 6-01 of radio-frequency amplifying tube V1. Tube V4 functions in both cases as a potentially controlled thermionic relay device and not as a mere coupling capacity.

By connecting antenna I to the input circuit 20 of Vi -rather than to the input circuit of V1, another advantage is realized, namely, the tuning of the receiver, as in the present embodiment by manipulation of condensers C1, C2 and C3, is entirely independent of the natural capacity of 25 the antenna system l-2. This feature is desirable in that the three tuning condensers mentioned may thus be calibrated-to wave lengths or frequencies and the calibrations will remain correct regardless of the size of the antenna to which the receiver may be connected. The advantage is particularly promlnent when all three tuning condensers are coupled to a single manual control, as indicated by the dotted line connecting the three variable tuning condensers of Fig. 1, since without the provision of antennacapacity isolation, the uni-control type of receiver is impracticable. f

A third advantage of the invention resides in its application to a receiver of the regenerative 40 or retroactive type, illustrated in Fig. 2. In such a receiver-a typical embodiment of which might comprise three vacuum tubes, the first a regenerat.ve detector, the second and third being audio-frequency amplifying tubes, the connec- 46 tion of the antenna to the grid or input circuit of the first audio-frequency amplifying tube would prevent to a very large'degree radiation from the receiver due to oscillating energy generated in the regenerating or oscillating detector. 50

Th's action is based upon the well-known-unidirectional characteristics. of the thermionic relay. v

The course of a radio signal through the receiver illustrated in Fig. 1 would be. as follows: 65

from antenna the received energy is applied to the input circuit of tube V4, which, acting as a thermionic relay for the radio-frequency energy, amplifies the signals and passes them into radio'- frequency output circuit ii-Ca. Impedance R, in this instance a resistance of about 3,000 ohms, inserted in the grid circuit of tube V4 between the secondary 4 of transformer T4 and the point at which antenna 1 is connected, is inserted for the purpose of obtaining the required high-frequency potential across the input circuit of tube V4. It can be seen that the desired result could be attained by substituting for the resistance indicated, a reactance coil tuned to any desired frequency, or, in the alternative, R could be a radio-frequency choke coil. These alternative modifications will be described in connection with Fig. 3. A. by-pass condenser C4 is preferably conby utilizing the reactance of inductance 4.

' to obtain the best operation, the distributed capacity of audio-transformer T4 should be small.

Coil 5 thus becomes the primary of the radiofrequency transformer T1, the secondary 6 of,

which is tuned by condenser C1. From tuned circuit 5C1 the signals pass through radiofrequency amplifying tube V1 through transformer T2, the secondary of which is tuned by condenser 02, into the second radio-frequency amplifying tube V2 and thence through coupling transformer T3, the secondary of which is tuned by condenser C3. From the plate to the filament of V: a by-pass condenser C7 is connected to by-pass unrectified radio-frequency currents across primary 3 of the first stage audio-frequency amplifying transformer T4. The rectified signals, now of audiofrequency, pass through transformer T4 into the first stage audio-frequency amplifying tube V4, through the second stage audio-frequency ampliiying transformer T5 into the second stage audiofrequency amplifying tube V5, which has connected in its output circuit a loud speaker or other indicating or translating device LS.

In order that undesirable impedance coupling may be prevented as far as possible, radio-frequency by-pass condenser C5 is connected across the filament battery, and condenser C6 is connected directly from the common point of connection of the primary windings of radio-frequency transformers T2 and T3 to the low-potential side of the filament system.

Referring now to Fig. 2, a modification of the arrangement of Fig. 1 is illustrated wherein a receiver comprising three vacuum tubes incorporates the advantages of the present invention. It will be observed that this modification is similar to the arrangement of Fig. 1, except that the present arrangement omits the two radio-frequency amplifying tubes V1 and V2, and in their stead a certain amount of radio-frequency amplification is attained thru the use of regeneration in the detector tube V3. present invention as applied to the circuit arrangement of Fig. 2 is essentially similar to "that previously explained; the reference characters of the two figures corresponding thruout. Tube V3 is caused in this instance to be regenerative thru The operation of the the medium of radio-frequency feed-back coil 9 shown as coupled to the secondary winding 6 of radio-frequency coupling transformer T1. The audio-frequency amplifying stage including tube V4 is shown in Fig. 2 to include a neutralizing con- 5 denser CH3 connected in exactly the same manner as neutralizing condensers Cnl and 0:12 of Fig. 1. The purpose in neutralizing the capacity coupling of this tube is, of course, the same as the purpose in neutralizing tubes V1 and V2 of Fig. l, 10 i. e., by neutralizing the inherent capacity effectively existing between the grid and plate electrodes of the tube the transfer of energy at radiofrequencies from the plate to the grid of tube V4 is prevented. Grid condenser II and grid leak I2 are included in the grid circuit of tube V: for the well-known purpose of detection. Grid leak l2 of Fig. 2 is, however, connected directly between the grid and filament of the detector tube, rather than in shunt to the grid condenser H as in Fig. l, in order that the grid condenser it may also function as a stopping condenser to insulate the grid of tube V; from the potential of the common plate-battery B.

The modification of Fig. 3 is essentially the same as that of Fig. 2, but differs therefrom in that instead of employing a radio-frequency coupling transformer T1 for feeding back the received radio signals from the output circuit of the first audio-frequency amplifyingtube V4 to the input circuit of the detector V3, as in Fig. 2,

a radio-frequency impedance R1 has been substituted. The radio-frequency signal voltages relayed by tube V; are in this instance built up across radio-frequency choke R1 and transferred to the tuned input circuit of the detector V3 thru the coupling condenser l3, which also functions to insulate the grid of the detector tube from the potential of the common plate-battery B in a manner similar to that of condenser E l of Fig. 2. As has been explained in connection with Fig. 1, in the eventthat the radio-frequency potentials are built up across an introduced impedance, such as R and R1, it is usually desirable that a by-pass condenser, such as C4 and I0 respectively,

be shunted around that portion of the coupling transformer which is connected in series with such introduced impedance element. However, as also explained above, in the event that the impedance element is omitted, the desired radiofrequency potentials may be established across the impedance of the windings of the coupling transformer, such as 4 and 7 respectively, in which case the by-pass condenser would be omitted. The impedance R1 of Fig. 3 has been drawn to represent a radio-frequency choke, altho, of course, a non-inductive resistance, similar to R, but of about ten thousand ohms, can be substituted. The capacity indicated in dotted lines as being connected across choke coil or reactance coil R1 is intended to represent either the natural distributed capacity of the winding or else an added capacity; the result of either or both of which would be to tune the impedance to a certain frequency or frequency range. However, in most cases an untuned impedance is to be preferred. While in the arrangement of Fig. 3, there is no neutralized tube connected between the regenerative detector and the antenna, nevertheless the arrangement shown will not radiate oscillations to any harmful extent because of the fact that such oscillations would be greatly attenuated thru the extremely small capacity existing between the grid and plate of audio-frequency amplifying tube V4. 7

In the circuitarrangements shown in Pig. 2 and Fig. 3, it is to be understood that one or more radio-frequency amplifying stages, which may or may not be neutralized, may be included after the manner shown in Fig. 1; and that, furthermore, any suitable low-frequency coupling device.

or devices, may be substituted to precede the tuned input circuit of said audio-frequency amplifying tube, impedance; means in said input circuit across which are established radio-fre-- quency potentials derived from said antenna system, radio-frequency coupling means connected between the output circuit of said audiofrequency amplifying tube and the input circuit of said radio-frequency amplifier, radio-frequency coupling means connected between the output circuit of said radio-frequency amplifier and the input circuit of said detector, audiofrequency coupling means connected between the output circuit of said detector and the input circuit of said audio-frequency amplifying tube,

and means for coupling a translating device with the output circuit of said audio-frequency amplifying'tube.

2. A radio receiver including a detector, the output circuit of said detector being coupled to the untuned. input circuit of an audio-frequency amplifying tube, and means including an im-'- pedance for connecting an untuned antenna system to said untuned input circuit of said amplifying tube, whereby said tube relays substantially aperiodically at both radio and audio frequencies. 4 3. In a wave signaling system, a detector coupled to the input circuit of an untuned audiofrequency amplifying tube, an aperiodic antenna system connected to said input circuit of said amplifying tube, and means for transferring radio-frequency currents from the output circuit of said amplifying tube to the input circuit of said detector, whereby saidtube relays substantially aperiodlcally at both radio and audio frequencies, and a translating device coupled to the output circuit of said audio-frequency amplifying tube.

, 4. A radio receiver including a detector, the

output circuit of said detector being coupled to the untuned-input circuit of an audio-frequency amplifying tube, means ,forconnecting an untuned antenna system to said input circuit of said amplifying tube, means included in the input circuit of said 'amplifyingtube and means included in the output circuit of said amplifying tube :across which radio-frequency potentials are established, means for transferring said radio-frequency potentials from the output circuit of said amplifying tube to the input circuit of said detector, and signal-translating means coupled to theoutput circuit of said amplifying tube.

5. In a radio receiver for high-frequency currents, a detector coupled to a low-frequency amplifying tube through a low-frequency transformer having primary winding and a secondary winding, an untuned antenna system connected to the untuned input circuit of said lowcies, and whereby said low-frequency amplifying 5 tube relays substantially aperiodically both low.- frequency currents and high-frequency currents. 6. A radio receiver including a multistage tuned radio-frequency vacuum tube amplifier,

a detector connected to the output circuit of said radio -frequency amplifier, means for neutralizing undesirable capacity coupling between the output and input circuits of the respective stages of said amplifier, an audio-frequency vacuum tube amplifier connected to the output of said detector, and means for conductively connecting an antenna to the'input circuit and to the grid electrode of a vacuum tube of said audio-frequency amplifier, the output circuit of said lastmentioned amplifier including a radio-frequency transformer the secondary of which is connected to the input circuit of said multistage radiofrequency amplifier.

7. A radio receiver including a multistage tuned radio-frequency vacuum .tube amplifier, 2 means for neutralizing undesirable capacity coupling between the output and input circuits of the'respective stages of said amplifier, a detector connected to the output circuit of said amplifier, an audio-frequency vacuum tube amplifier connected to the output of said detector, the output circuit of said audio-frequency amplifier including the primary winding of a radio-frequency transformer comprising a primary winding and a secondary winding, the secondary winding being connected to the input circuit of said radio-frequency amplifier, an impedance connected in series with the grid of said audio-frequency amplifying tube, and means for conment connected in series with at least a portion I of said coupling device in circuit between antense. and ground, means whereby a substantially untuned antenna may be connected at a point between the grid electrode of said tube and the said series-connected coupling device and impedance element, a low-impedance path for radio-frequency currents across terminals of said coupling device whereby the antennaground system may be substantially aperiodic, means connected in the output circuit of said tube across which radio-frequency. potentials may be established, and means for' transferring radio-frequency oscillations resulting from said potentials fromithe output circuit of said tube to the input circuit of said detector. 9. 'A radio receiver including a detector coupled to an amplifying tube by means ofan element comprising a vsubstantially non-inductive resistance connected in series with a portion v of saidcoupllng device in'series between antenna and ground, said portion of said coupling device being connected between said impedance element and ground, means whereby a substantially untuned antenna may be'connected between the grid electrode of said amplifying tube and said impedance element, a low-impedance path for radio-frequency currents comprising a condenser connected across said portion of said coupling device whereby the antenna-ground system may be substantially aperiodic, means including a radio-frequency transformer having atuned secondary winding for transferring radiofrequency oscillations from the output circuit of said tube to the input circuit of said detector, and means whereby an audio-frequency translating device may be linked with the output circuit of said amplifying tube.

I 10. In a radio signaling system, a detector coupled by means of an audio-frequency transformer to the untuned input circuit of an audiofrequency amplifying tube having a grid electrode, said transformer including a primary winding and a'secondary winding, and. means whereby an untuned antenna may be effectively connected between one end of said secondary winding and the grid electrode of said tube, whereby radio-frequency signals thruout a wide band of frequencies may be relayed by said tube, and means including an impedance associated with the output circuit of said tube for transferring radio-frequency signals therein to the input circuit of said detector.

11. A receiver for radio-frequency signals, comprising a portion having a circuit tunable to certain desired radio-frequency signals, a second portion including an audio-frequency amplifier having no circuits tuned or tunable to said radio-frequency signals, and means for collecting radio-frequency signals and delivering them to the first-mentioned portion thru said amplifier of the second-mentioned portion.

12. A receiver for radio-frequency signals, comprising a portion including a radio-frequency amplifying vacuum tube, said portion having circuits tunable to certain desired radio-frequency signals, a second portion including an audiofrequency amplifying vacuum tube having no circuits tuned or tunable to said radio-frequency signals, a substantially aperiodic antenna system for collecting said radio-frequency signals,

and means for delivering said signals to said first-.

mentioned portion thru said audio-frequency amplifying tube of said second-mentioned portion.

13. A receiver for radio-frequency signnls comprising a portion including a radio-frequency amplifier and a detector coupled thereto, said amplifier and detector including a plurality of vacuum tubes, said portion having circuits tunable to certain desired radio-frequency signals, a second portion including an audio-frequency amplifying vacuum tube having no circuits tuned or tunable to said radio-frequency signals, a substantially aperiodic antenna system for collecting said signals and a plurality of means including a tuned radio-frequency transformer for delivering said signals to the radio-frequency amplifler of said first-mentioned portion thru said audio-frequency amplifying vacuum tube of said second-mentioned portion.

14. A radio receiving system comprising the combination with an absorption structure, of a receiving set comprising a plurality of thermionic devices in cascade and at least one of which amplifies the absorbed radio frequency energy, devices associated with. said thermionic devices for tuning circuits in cascade to the same wave lengths throughout the same range of wave lengths, and means for insuring that the adjustable elements of said tuning devices shall partake of equal extents of movement in adjusting from resonance for one wave length to resonance for another wave length comprising a thermionic amplifier whose output circuit is coupled to the input circuit --of the first of said thermionic devices and whose grid and cathode are directly connected with said absorption structure.

15. A radio receiving system comprising the combination with an absorption structure, of a receiving set comprising a plurality of thermionic devices in cascade and at least one of which amplifies the absorbed radio frequency energy, devices associated with said thermionic devices for tuning circuits in cascade to the same wave lengths, throughout the same range of wave lengths, means for insuring that the adjustable elements. of said tuning devices shall partake of equal extents of movement in adjusting from resonance for onewave length to resonance for another wave length comprising a thermionic amplifier whose output circuit is coupled to the input circuit of the first of said thermionic devices and whose grid and cathode are directly connected with said absorption structure, and an inductance connected between said grid and cathode and having such a magnitude as substantially to prevent passage therethroughof received radio frequency energy and to by-pass low frequency energy from said grid.

16. A radio receiving system comprising the combination with an absorption structure, of a plurality of circuits in cascade, devices associated with said circuits for tuning them to the same wave lengths throughout the same range of wave lengths, and means for insuring thatthe adjustable elements of said tuning devices shall partake of equal extents of movements in adjusting from resonance for one wave length to resonance for another wave length comprising a vacuum tube interposed between the first of said tuned circuits and said absorption structure, the input of said tube and said absorption structure constituting an input system the magnitudes of whose inductance and capacity are substantially constant. 1

17. A radio receiving system comprising the combination with an absorption structure of a plurality of circuits in cascade, a variable condenser for tuning each of said circuits, and means for insuring that said condensers shall partake of equal extents of adjustments from resonance for one wave length to resonance for another wave length comprising a vacuum tube interposed between the first of said tuned circuits and said absorption structure, said absorption structure and the input of said tube constituting an input system the magnitudes of whose inductance and capacity are substantially constant.

18. A radio receiving system comprising the combination with an absorption structure, of a plurality of circuits in cascade, means for tuning each of said circuits, a vacuum tube interposed between the first of said tuned circuits and said absorption structure, the input of said tube and said absorption structure constituting an input system the magnitudes of whose inductance and capacity are substantially constant throughout the wave length range of the received energy, and so related to said tunable circuits as to permit equal movements of their tuning devices in adjusting from resonance for one wave length to resonance for another wave length, and an inductance connected between the grid and cathode of said tube and having such magnitude as substantially to prevent passage therethrough of received radio frequency energy and to bypass low frequency energy from said grid.

19. The method of tuning cascaded circuits throughout a frequency range of energy received 20. In a radio receiving system comprising a plurality of .thermionic devices in cascade and at least one of which amplifies the received radio frequency energy, devices associated with said thermionic devices for'tuning circuits in cascade to the same wave lengths throughout the same.

range of wave lengths, and a thermionic amplifier disposed in a later stage with respect .to said first named thermionic devices, the method of avoiding influence by the characteristics of the absorption structure upon the first ofsaid tuned circuits, which comprises impressing the received radio frequency energy upon the input circuit of said last namedthermionic device, and impressing the radio frequency component of the current in the anode circuit of said last named thermionic device upon the input circuit of the first of said first named thermionic devices. a

21. A radio receiving system comprising the combination with an absorption structure, of a plurality of circuits in cascade, devices associated with said circuits for tuning them to the same wavelengths throughout the same range of wavelengths, means mechanically coupling the adjustable elements of said tuning devices for eifecting their adjustment in unison to equal extents, and means for insuring that the adjustable elements of said tuning devices shall partake of equal extents of movement in adjusting from resonance for one wavelength to resonance 1 for another wavelength comprising a vacuum tube interposed between the first of said tuned circuits and said absorption structure, the input of said tube and said absorption structure constituting an input system the magnitudes of whose inductance and capacity are substantially constant.

22. A radio receiving system comprising the combination with an absorption structure, of a plurality of circuits in cascade, a variable condenser for tuningeach of said circuits, equal increments of adjustment of each of said condensers eflecting equal increments of its capacity, means mechanically coupling the adjustable elements of said condensers for effecting their adjustment in unison to equal extents, and .means for insuring that said condensers shall partake of equal extents of. adjustment from resonance for one wavelength to resonance for another wavelength comprising a vacuum tube interposed be-' tween the first of said tuned circuits and said absorption structure, said absorption structure and the input of said tube constituting an input 1 system the magnitudes of whose inductance and capacity are substantially constant.

23. A radio receiving system comprising the combination with on absorption structure, of a receiving set comprising a plurality of thermionic devices in cascade and at least one of which amplifies the radio frequency energy, condensers associated with said thermionic devices for tuning to the same wavelengths throughout a range of wave lengths, each of said condensers comprising rotor and stator elements of such character that the increments of capacity are equal for equal increments or adjustment of the rotor with respect to the stator, and means for insuring that the rotors of said condensers shall partake of equal extent of movement in adjusting from resonance for one wavelength to resonance for an- 5 other wavelength comprising a three-electrode thermionic amplifier whose output circuit is coupled to the input circuit of the first of said ther mionic devices and whose input is operativelyre lated to said absorption structure, theinput of 1 said last namedthermionic device and said ab-f' sorption structure constituting an inputsystem the magnitudes of whose inductance and capacity v are substantially constant. j v, 24. A radio receiving system comprising the combination with an absorption structure, of a receiving set comprising a plurality of thermionic devices in cascade and at least one of which amplifies the radio frequency energy, condensers associated with said thermionic devices for tun- '20 ing to the same wavelengths throughout a range of wavelengths, each of said condensers comprising rotor and stator elements of such character that the increments of capacity are equal for equal increments of adjustment of the rotor with respect to the stator, means mechanically coupling the rotors of said condensers for effecting their adjustment in unison to equal extents, and means for insuring that the circuits tunable by said condensers shall be simultaneously resonant 0 to the same wavelength comprising a three-electrode thermionic amplifier interposed between the first tuned circuit and-said absorption structure for reactively isolating themfrom each other, the input of said last named thermionic device 5 and said absorption structure constituting an input system the magnitudes of whose inductance and capacity are substantially constant.

25. A radio receiving system comprising the combination with an' absorption structure, of a 40 receiving set comprising a plurality of thermionic devices in cascade and atleast one of which amplifies the radio frequency energy absorbed by said absorption structure, devicesassociated with said thermionic devices for tuning to, the

same wavelengths throughout the same range of wavelengths, and means for insuring that theadju'stable elements" of said tuning devices shall partake of equal extents of movement in adjusting from resonance for one wavelength to mionic devices, devices associated with said thermionic devices for tuning to the same wavelengths throughout the same range of wavelengths, means for insuring that the adjustable elements of said tuning devices shall partake of equal extents of movement in adjusting from resonance for one wavelength to resonance for another wavelength comprising a thermionic amplifier whose output circuit is coupled to the first of said thermionic devices, and a transformer operatively associated with said absorption structure having a winding the terminals of which are connected to the input electrodes of said thermionic amplifier.

27. In a radio receiving system comprising a plurality of thermionic devices in cascade and at least one of which amplifies the received radio frequency energy, devices associated with said thermionic devices for tuning circuits in cascade to the same wavelengths throughout the same range of wavelengths, and a thermionic amplifier disposed in a later stage with respect to said first named thermionic devices, the method of substantially preventing the influence by the characteristics of the absorption structure upon tuning which comprises impressing the received radio frequency energy upon the input circuit of said last named thermionic device, im pressing the radio frequency energy so amplified upon the input circuit of the first of said first named thermionic devices, and simultaneously amplifying by the last named thermionic device the audio frequency current representative of the audio frequency modulation of the received energy.

28. A radio receiving system comprising a radio frequency amplifier, a thermionic detector whose input circuit is influenced by the current in the I said radio frequency amplifier by said absorption structure comprising means for impressing the energy absorbed by said absorption structure upon the input circuit of said audio frequency amplifier, and a coupling between the output circuit of said audio frequency amplifier and the input circuit of said radio frequency amplifier.

29. A radio receiving system comprising a plurality of thermionic devices in cascade, at least one of which is a radio frequency amplifier, a device for tuning the input circuit of each of said thermionic devices, said tuning devices being similar to each other toeffect substantially equal changes in tuning when their adjustable tuning elements are adjusted to equal extents, an audio frequency thermionic amplifier distinct from and succeeding said thermionic devices, a radio frequency coupling between the output circuit of said audio frequency amplifier and the input. circuit of the first of said first named thermionic devices, and absorption structure opera- 2,o14,oso

tively related to the input circuit of said audiofrequency amplifier, to avoid the influence of 'coupling between the output circuit of said amplifier and the first of said tunable circuits, and 15 absorption structure operatively related to the input circuit of said audio frequency amplifier to avoid the influence of the characteristics of said structure upon tuning.

31. In a radio receiving system '[comprising a 20 plurality of circuits in cascade,fdevices associated therewith to tune the circuits to the same wavelengths throughout the same range of wavelengths, and a thermionic device succeeding the tunable circuits, the method of substantially preventing the influence by the characteristics of the absorption structure upon tuning which comprises impressing the received radio frequency energy upon the input circuit of said thermionic device, impressing the radio frequency energy in the output circuit of said device upon the first of said tuned circuits, tuning said circuits to select from said energy a desired signal, and amplifying said selected signal by said thermionic device. 32. A radio receiving system comprising the combination with an absorption structure, of a plurality of circuits in cascade, the inductance of each of said circuits having fixed magnitude and comprising a winding of a coupling trans- 40 former, a single variable condenser constituting the sole means for tuning each of said circuits, and means for insuring that the adjustable elements of said condensers shall be independent of the electrical constants of said absorption structure in adjusting from resonance for one wave length to resonance for another wave length comprising a vacuum tube interposed between the first of said tuned circuits and said absorption structure, said absorption structure and the input of said tube constituting an input system the magnitudes of whose inductance and capacity are substantially constant.

A. MACDONALD. 

